Never consider your opponent to be weaker than you.

Answer: Option D

Opponent : someone who competes with or opposes another in a contest, game, or argument.
Friend : a person with whom one has a bond of mutual affection, typically one exclusive of sexual or family relations.
Rival : a person or thing competing with another for the same objective or for superiority in the same field of activity.
Antagonist : a person who actively opposes or is hostile to someone or something; an adversary.
Ally : side with or support.

Antonym of Opponent is Ally